This invention relates generally to a three-dimensional (xe2x80x9c3Dxe2x80x9d) measurement/digitization system and method, and in particular to a portable 3D digitization system and method which facilitate acquisition of data relating to 3D profiles of objects for subsequent computer-aided processing and reproduction of the 3D profiles of objects by shape digitizing.
Speed, accuracy, and portability have been recurrent and difficult to achieve goals for devices that scan, measure or otherwise collect data about 3D objects for purposes such as reproduction. With the advent of computers, such devices have useful application in many fields, such as digital imaging, computer animation, topography, reconstructive and plastic surgery, dentistry, internal medicine, rapid prototyping, and other fields. These computer-aided systems obtain information about an object and then transform the shape, contour, color, and other information to a useful, digitized form.
The technology currently available for shape digitizing falls into two different but related groups: mechanical systems and optical systems. All systems within those two general categories struggle with the basic criteria of speed, accuracy, portability and ability to digitize the color texture image of an object.
A mechanical system acquires data about an object through the use of a probe that has a sensitive tip. The mechanical system scans an object by moving its probe tip across the object""s surface and taking readings. Generally, the probe connects to a mechanical arm, and the system tracks the probe""s position in space using angle measuring devices as the arm moves. The system calculates the position of the probe with coordinates known from the angle measuring devices.
Although mechanical systems scan with generally high accuracy, the rate at which a mechanical system acquires the data is relatively slow and can take several hours for scanning. A typical mechanical system measures only one point at a time, and no information is obtained about the material properties of the object such as its color.
As an alternative to mechanical systems, there are several types of optical object shape digitizers which fall into two basic categories: systems based on triangulation and alternative systems. A triangulation system projects beams of light on an object and then determines three-dimensional spatial locations for points where the light reflects from the object. Ordinarily, the light source is located at a certain distance from the light detector, and relative positions of the components and the direction of the light beam need to be known. A single dot system projects a single beam of light which, when reflected, produces a single dot of reflection. A scan line system sends a plane of light against the object which projects on the object on a line and reflects as a curvilinear-shaped set of points describing one contour line of the object. The location of each point in that curvilinear set of points can be determined by trigonometric triangulation.
Some single dot optical scanning systems use a linear reflective light position detector to read information about the object. In such systems a laser projects a dot of light upon the object. The linear reflected light position detector occupies a position relative to the laser which allows the determination of a 3D location for the point of reflection. A single dot optical scanner with a linear reflected light position detector can digitize only a single point at a time. Thus, a single dot optical scanning system, like mechanical system described above, is relatively slow in collecting a full set of points to describe an object. Single dot optical scanners are typically used for applications such as industrial engineering. The digitizing speed is usually limited by the mechanics of the scanning system, i.e., the moving and positioning of the laser beam. A scanning head can be mounted on a high-precision, but costly, positioning system to take a digitized image of the object""s shape with generally good accuracy. However, because of the high cost, slow speed and difficulty of obtaining material properties such as colored texture, single dot optical scanners find generally only limited applications.
Scan line systems offer one solution to the speed bottleneck of single point triangulation system. Those systems typically employ a 2D imager, such as a charge coupled device (CCD) camera, for signal detection. The system projects a light plane (i.e., a laser stripe) instead of just one dot and read the reflection of multiple points depicting the contour of an object at a location that is at a distance from the CCD camera and from which the position can be triangulated. Some embodiments of the scan line-type system attach the CCD camera to a rotating arm or a moving platform. During scanning, either the object moves on a known path relative to the camera and laser, or the camera and laser, together, move around the object. In any case, such systems usually depend on this type of fixed rotational movement and typically use a bulky, high-precision mechanical system for positioning. Because of the use of mechanical positioning devices, resealing flexibility can be very limited, e.g., a scanner designed for objects the size of a basketball may not be useful for scanning apple-sized objects.
Some laser stripe triangulation systems currently available are further limited because the laser stripe stays at a fixed angle relative to the camera, and the system makes its calculations based on the cylindrical coordinates of its rotating platform. The mathematical simplicity in such a projection system complicates the hardware portion of these devices as they typically depend on the rotational platform mentioned. Also, the simplified geometry does not generally allow for extremely refined reproduction of topologically nontrivial objects, such as objects with holes in them (e.g., a tea pot with a handle). Full realization of triangulation scanning with a non-restrictive geometry has not been achieved in the available devices.
Apart from optical triangulation systems (single dot or structured line systems), there are alternative optical scanning systems which present a scanning solution different from those employing triangulation techniques. Range meters, depth-from-focus and multi-camera systems ate among those categorized as xe2x80x9calternativexe2x80x9d systems. Range meter systems typically use a pulsed laser and mechanical scanning techniques to project a dot laser across then measure the time or phase delay of the reflected signal. As range meter systems typically incorporate a single dot method of data collection, they are intrinsic to single-point scanners, and they typically do not acquire material properties of the object.
Another type of alternative scanning system is a stereoscopic system which uses several CCD cameras located at known distances from each other. The captured images are processed with a pattern recognition system which finds matching points in different images of the object, thereby obtaining the shape/contour information. One advanced stereoscopic system uses 6 high-resolution CCD cameras. Since matching points can not be identified on flat and texture-less parts of the object a special grid needs to be projected on the object to facilitate geometry reconstruction. In spite of that, data omissions frequently occur, and thus the method is not very reliable since the quality depends on the material reflective properties.
In the depth-from-focus method two images of the object are acquired with cameras focused to focal planes located closer and further away than the object. By comparing the defocused images the depth information can be obtained. To facilitate the depth reconstruction a special checkerboard grid is typically projected on the object. The method suffers from the problems reverse to the problems of stereoscopic imaging: the objects with rich texture can not be reliably processed. Also, the technique, similar to the stereoscopy, results usually in low geometric quality of the data, while the equipment incorporates at least two cameras and a special light projector, i.e., it is rather complex.
Thus, for devices that scan, measure or otherwise collect data about the geometry and material properties of an object, it would be a substantial advance if a digitizer could be created that could rapidly gather accurate data concerning a 3D object. It would also be an advance if the device would be simple in manufacturing and would be based on one of the mass-produced hardware architectures such are digital camera chip sets. Another advance would be if the device would capture the texture image of the object and determine object""s material properties such as diffuse and specular reflection coefficients. Furthermore, it would be an advance if the device has no moving parts.
It is an object of the present invention to provide a 3D measurement/digitizing system which is capable of rapid gathering of data relating to 3D profile of a measured object.
It is another object of the present invention to provide a 3D measurement/digitizing system which illuminates an object with a special kind of structured light pattern and records the shape of the reflected points of light by means of an image collector.
It is another object of the present invention to provide a 3D measurement/digitizing system which utilizes a triangulation technique that does not depend on the fixed direction of the light source relative to the camera.
It is another object of the present invention to provide a 3D measurement/digitizing system which includes a light projecting system that projects both structured light and uniform illumination light from the same apparent source or apparent location.
The present invention provides a high-speed, accurate and portable system and method for rapidly measuring objects and processing the shape, contour, color and material properties it collects for display, graphic manipulation, model building and other uses. Because the basic information about the object is obtained in rapid fashion, the invention is particularly suited to scan and measure objects which cannot easily stay motionless, such as human or animals. The mechanical and data processing features of the present invention permit the collected data to be processed with high accuracy and photo-realism.
The present invention also provides a system for illuminating an object with a special kind of structured light pattern, recording the shape of the reflected points of light by means of an image collector (such as a camera), and, by a triangulation technique that does not depend on the fixed direction of the light source relative to the camera, reconstructing the 3D shape of the object through a computer using the data points collected from the reflection of the structured light pattern. With the collected data points, a user can, inter alia, create, display and manipulate an image of the 3D object on a computer, physically reproduce the object (through computer controlled milling machines, stereolithography or digital holography), compress the data for easy transmission (such as over the Internet), or use the data in graphic manipulation systems (such as in 3D computer games).
The present invention also provides embodiments which are portable and can also be implemented using components which are readily available. A representative embodiment of the portable scanning system does not require a computer as part of the system because data processing contemporaneous with the data collection is obviated in this embodiment. Instead, the portable system stores in the storage media several images of the objects with different illumination patterns. The data is subsequently processed, at any desired time, by a computer system which applies data processing routines, i.e., the model building algorithms which provide 3D surface generation. It should be noted, however, processing of the data collected using the portable scanning system according to the present invention need not be limited to the specific data-processing routines described herein.
The digitization system according to the present invention utilizes the principle of optical, or geometrical, triangulation. While producing the quality of digitizing similar to the quality of laser-based triangulation sensors, the digitizer according to the present invention does not employ any moving parts, and it can be implemented completely with standard components of mass-produced digital cameras. The data acquisition according to the present invention is simplified to acquiring of only two or, optionally, four images of the object. Thus, the digitization speed is intrinsically superior to the scanning rate of laser-based scanners where a large number of images typically need to be acquired and processed.
Another feature of the present invention is that the light source projects both structured light and uniform illumination light from the same apparent source, and that allows for numerical normalization of the images. Such normalization increases consistency in quality of digitizing colored objects and also reduces the dependence on ambient light illumination.
An important feature of the structured light pattern according to the present invention is that the pattern consists of several stripes which have a linear slope of light intensity profile. During processing, not only the centers of the stripes are found, but also the data points are identified on the slopes of the stripes. The actual number of the stripes depends on the dynamic range of the camera. Thus the method utilizes not only pixel resolution of the camera, but also its dynamic-range for increasing the quality of digitization. Physically, 3D coordinates can be obtained for all pixels of the imager, i.e. it is not limited to the number of projected stripes.
According to the present invention, one or several stripes of the structured light pattern have different color than other stripes. Such a stripe can be easily distinguished from other stripes during image processing. Once such a stripe is identified, the data processing steps for obtaining the 3D profile of the scanned object follows the 3D-profile-generation algorithms used with the 3D scanning system described in U.S. patent application Ser. No. 08/620,689 filed on Mar. 21, 1996 by A. Migdal, M. Petrov and A. Lebedev, which application is explicitly incorporated herein by reference.
The present invention also provides a method of precise determination of material specular and diffusive reflection properties. It is made possible because the object is illuminated by spot-like light sources located at known distances from the camera.